Numerical modeling of the mechanical-chemical coupling effects of acid fracturing on carbonate geothermal reservoir: A case study of the Well D22 in Xiong'an New Area, China

Acid fracturing serves as a key stimulation technique for enhancing productivity in carbonate geothermal reservoirs by addressing permeability limitations and production decline. However, geothermal applications lack standardized protocols for this technology, requiring specific study of reservoir responses. This study established a coupled thermo-hydro-mechanical-chemical model through integration of fracture mechanics and reactive transport modules within TOUGH2's multiphysics framework. Utilizing actual field data from Well D22 in China's Xiong'an New Area and acid fracturing measurements, a permeability evolution model was developed to analysis spatiotemporal reservoir characteristics under combined mechanical-chemical effects during stimulation processes. The result indicates that stimulation effectively enhanced permeability throughout the fractured zone, with fracture orientation inducing anisotropic improvement. Permeability demonstrated a more pronounced improvement along the X -direction where the stimulated region extended approximately 75 m, compared to 25 m along the Y -direction. The largest scale of permeability enhancement and fracture damage occurred during Stage 2, with permeability enhancement resulting from fracture expansion and propagation under high fluid pressure and acid etching. Permeability enhancement due to fracture expansion gradually diminished as fluid pressure recovered, while those due to fracture propagation were maintained after injection ceased. Acid injection primarily improved near-wellbore reservoir permeability (radius <25 m) without substantially expanding the treatment zone. The above findings provide valuable insights for optimizing acidizing treatment designs and informing well placement strategies in multi-well geothermal reservoirs. • A coupled thermo-hydro-mechanical-chemical model was established through integration of fracture mechanics and reactive transport modules within TOUGH2's multiphysics framework. • Utilizing actual field data from Well D22 in China's Xiong'an New Area and acid fracturing measurements, a permeability evolution model was developed to analysis spatiotemporal reservoir characteristics under combined mechanical-chemical effects during stimulation processes.